Light and TelescopesChapter 6
In the early chapters of this book, you looked at the sky the way ancient astronomers did, with the unaided eye. In this chapter, you will see how modern astronomers use telescopes and other instruments to gather and focus light and its related forms of radiation. That will lead you to answer five essential questions about the work of astronomers:
• What is light?
• How do telescopes work, and how are they limited?
• How do astronomers record and analyze light?
• Why do astronomers use radio telescopes?
• Why must some telescopes go into space?
Guidepost
Astronomy is almost entirely an observational science, so astronomers must think carefully about the limitations of their instruments. That will introduce you to an important question about scientific data:
• How do we know? What limits the detail you can see in an image?
Fifteen chapters remain, and every one will discuss information gathered by telescopes.
Guidepost (continued)
I. Radiation ( 輻射 ): Information from SpaceA. Light as a Wave and a ParticleB. The Electromagnetic Spectrum ( 電磁光譜 )
II. Optical TelescopesA. Two Kinds of TelescopesB. The Powers of a TelescopeC. Buying a TelescopeD. New-Generation TelescopesE. Interferometry ( 干涉儀 )
III. Special InstrumentsA. Imaging SystemsB. The Spectrograph
Outline
IV. Radio TelescopesA. Operation of a Radio TelescopeB. Limitations of the Radio TelescopeC. Advantages of Radio Telescopes
V. Astronomy from SpaceA. The Ends of the Visual SpectrumB. Telescopes in SpaceC. Cosmic Rays
Outline (continued)
Light and Other Forms of Radiation
• The Electromagnetic Spectrum
In astronomy, we cannot perform experiments with our objects (stars, galaxies, …).
The only way to investigate them, is by analyzing the light (and other radiation) which we observe from them.
Light as a Wave (1)
• Light waves are characterized by a wavelength and a frequency f.
f = c/
c = 300,000 km/s = 3*108 m/s
• f and are related through
Light as a Wave (2)
• Wavelengths of light are measured in units of nanometers (nm) or Ångström (Å):
1 mm = 10-3 m
1 μm = 10-6 m
1 nm = 10-9 m
1 Å = 10-10 m = 0.1 nm
Visible light has wavelengths between 4000 Å and 7000 Å (= 400 – 700 nm).
Wavelengths and Colors
Different colors of visible light correspond to different wavelengths.
Light as Particles
• Light can also appear as particles, called photons (explains, e.g., photoelectric effect).
E = h*f
h = 6.626x10-34 J*s is the Planck constant.
The energy of a photon does not depend on the intensity of the light!!!
• A photon has a specific energy E, proportional to the frequency f:
Intensity: 在單位時間 單位面積 單位角度 單位頻率 內 所接收 ( 放出 ) 的能量
The Electromagnetic Spectrum
Need satellites to observe
Wavelength
Frequency
High flying air planes or satellites
The Electromagnetic Spectrum
Optical TelescopesAstronomers use
telescopes to gather more light from
astronomical objects.
The larger the telescope, the more
light it gathers.
Refracting/Reflecting Telescopes折射式望遠鏡
Refracting Telescope:
Lens focuses light onto the focal plane
反射式望遠鏡Reflecting Telescope:
Concave Mirror focuses light onto the focal
planeAlmost all modern telescopes are reflecting telescopes.
Focal length
Focal length
Secondary OpticsIn reflecting telescopes: Secondary
mirror, to re-direct the light path towards
the back or side of the incoming
light path.
Eyepiece: To view and
enlarge the small image produced in
the focal plane of the
primary optics.
Disadvantages of Refracting Telescopes
• Chromatic aberration ( 色差 ): Different wavelengths are focused at different focal lengths (prism effect).
Can be corrected, but not eliminated by second lens out of different material
• Difficult and expensive to produce: All surfaces
must be perfectly shaped; glass must be flawless;
lens can only be supported at the edges
The Powers of a Telescope:Size Does Matter
1. Light-gathering power (or collecting power): Depends on the surface area A of the primary lens / mirror, proportional to diameter squared:
A = (D/2)2
D
The Powers of a Telescope (2)
2. Resolving power: Wave nature of light => The telescope aperture produces fringe rings that set a limit to the resolution of the telescope.
min = 1.22 (/D)
Resolving power = minimum angular distance min between two objects that can be separated.
For optical wavelengths, this gives
min = 11.6 arcsec / D[cm]
min
Seeing
Weather conditions and turbulence in the atmosphere set further limits to the quality of astronomical images.
Bad seeing Good seeing
The Powers of a Telescope (3)
3. Magnifying Power = ability of the telescope to make the image appear bigger.
The magnification depends on the ratio of focal lengths of the primary mirror/lens (Fo) and the eyepiece (Fe):
M = Fo/Fe
A larger magnification does not improve the resolving power of the telescope!
The Best Location for a Telescope
Far away from civilization – to avoid light pollution
The Best Location for a Telescope (2)
On high mountain-tops – to avoid atmospheric turbulence ( seeing) and other weather effects
Paranal Observatory (ESO), Chile
Traditional Telescopes (1)
Traditional primary mirror: sturdy, heavy to avoid distortions
Secondary mirror
Traditional Telescopes (2)
The 4-m Mayall
Telescope at Kitt Peak
National Observatory
(Arizona)
4 m4 m
Advances in Modern Telescope Design (1)
1. Lighter mirrors with lighter support structures, to be controlled dynamically by computers
Floppy mirror
Segmented mirror
Modern computer technology has made significant advances in telescope design possible:
Adaptive OpticsComputer-controlled mirror support adjusts the mirror surface (many times per second) to compensate for distortions by atmospheric turbulence
Advances in Modern Telescope Design (2)
2. Simpler, stronger mountings (“Alt-azimuth mountings”) to be controlled by computers
Equatorial mounting: 赤道儀 Alt-azimuth mounting: 經緯儀
Examples of Modern Telescope Design (1)
Design of the Large Binocular Telescope (LBT)
Examples of Modern Telescope Design (2)
8.1-m mirror of the Gemini Telescopes
The Very Large Telescope (VLT)
InterferometryRecall: Resolving power of a telescope depends on diameter D:
min = 1.22 /D.
This holds true even if not the entire surface is filled out.
• Combine the signals from several smaller telescopes to simulate one big mirror
Interferometry
CCD ImagingCCD = Charge-coupled device
• More sensitive than photographic plates• Data can be read directly into computer memory, allowing easy electronic manipulations
Negative image to enhance contrasts
False-color image to visualize brightness contours
• 1966, Charles K. Kao made a discovery that led to a breakthrough in fiber optics. He carefully calculated how to transmit light over long distances via optical glass fibers. With a fiber of purest glass it would be possible to transmit light signals over 100 kilometers, compared to only 20 meters for the fibers available in the 1960s.
• In 1969 Willard S. Boyle and George E. Smith invented the first successful imaging technology using a digital sensor, a CCD (Charge-Coupled Device). The CCD technology makes use of the photoelectric effect, as theorized by Albert Einstein and for which he was awarded the 1921 year's Nobel Prize. By this effect, light is transformed into electric signals. The challenge when designing an image sensor was to gather and read out the signals in a large number of image points, pixels, in a short time. The CCD is the digital camera's electronic eye. It revolutionized photography, as light could now be captured electronically instead of on film.
See Press Release ( 新聞稿 )
http://nobelprize.org/nobel_prizes/physics/laureates/2009/press.html
Astronomers who won Astronomers who won Nobel Physics PrizesNobel Physics Prizes
• 1967 Bathe - 核反應理論應用在恆星如何產生能量• 1974 Ryle - 無線電干涉儀在天文上的應用 Hewish - 發現脈衝星 ( 或稱”波霎” pulsar) (1967)
• 1978 Penzias & Wilson - 發現宇宙微波背景輻射 (1964)
• 1983 Chandrasekhar - 恆星的結構與演化 (理論) Fowler - 宇宙中元素的形成與演化(理論 + 實驗)• 1993 Taylor & Hulse - 發現脈衝雙星 , 驗證重力波理論• 2002 Davis & Koshiba - 太陽微中子的探測 Giacconi - 建立觀測宇宙 X 射線的望遠鏡• 2006 Mather & Smoot - 精確測量宇宙微波背景輻射與不均向
性
CCD
see Wikipedia for animationhttp://en.wikipedia.org/wiki/File:CCD_charge_transfer_animation.gif
Very Large TelescopesVery Large Telescopes
optical fibers are used to observe point sources in astronomy-- one fiber corresponds to one object
The SpectrographUsing a prism (or a grating), light can be split up into different wavelengths
(colors!) to produce a spectrum.
Spectral lines in a spectrum tell us about the chemical composition and other properties of the observed object
Radio AstronomyRecall: Radio waves of ~ 1 cm – 1 m also penetrate the Earth’s atmosphere and can be
observed from the ground.
Radio Telescopes
Large dish focuses the energy of radio waves onto a small receiver (antenna)
Amplified signals are stored in computers and converted into images, spectra, etc.
Radio MapsRadio maps are often color coded:
colors in a radio map can
indicate different intensities of the radio emission from different
locations on the sky.
Like different colors in a seating chart of a baseball stadium may indicate different seat prices, …
Radio InterferometryJust as for optical telescopes, the resolving power of a radio telescope is min = 1.22 /D.
For radio telescopes, this is a big problem: Radio waves are much longer than visible light.
Use interferometry to improve resolution!
Radio Interferometry (2)The Very Large Array (VLA): 27 dishes are combined to simulate a large dish of 36 km in diameter.
Even larger arrays consist of dishes spread out over the entire U.S. (VLBA = Very Long Baseline Array) or even the whole Earth (VLBI = Very Long Baseline Interferometry)!
The Largest Radio Telescopes
The 100-m Green Bank Telescope in Green Bank, WVa.
The 300-m telescope in Arecibo, Puerto Rico
Science of Radio Astronomy
Radio astronomy reveals several features, not visible at other wavelengths:
• Neutral hydrogen clouds (which don’t emit any visible light), containing ~ 90 % of all the atoms in the Universe
• Molecules (often located in dense clouds, where visible light is completely absorbed)
• Radio waves penetrate gas and dust clouds, so we can observe regions from which visible light is heavily absorbed.
Infrared Astronomy
However, from high mountain tops or high-flying air planes, some infrared radiation can still be observed.
NASA infrared telescope on Mauna Kea, Hawaii
Most infrared radiation is absorbed in the lower atmosphere.
Infrared cameras need to be cooled to very low temperatures, usually using liquid nitrogen.
NASA’s Space Infrared Telescope (Spitzer)
Infrared light with wavelengths much longer than visible light (“Far Infrared”) can only be
observed from space.
Ultraviolet Astronomy• Ultraviolet radiation with < 290 nm is
completely absorbed in the ozone layer of the atmosphere.
• Ultraviolet astronomy has to be done from satellites.
• Several successful ultraviolet astronomy satellites: IUE, EUVE, FUSE
• Ultraviolet radiation traces hot (tens of thousands of degrees), moderately ionized gas in the Universe.
The Hubble Space Telescope
• Avoids turbulence in the Earth’s atmosphere
• Extends imaging and spectroscopy to (invisible) infrared and ultraviolet
• Launched in 1990; maintained and upgraded by several space shuttle
service missions throughout the 1990s and early 2000’s
Gamma-Ray AstronomyGamma-rays: most energetic electromagnetic radiation;
traces the most violent processes in the Universe
The Compton Gamma-Ray Observatory
X-Ray Astronomy• X-rays are completely absorbed in the atmosphere.• X-ray astronomy has to be done from satellites.
NASA’s Chandra X-ray Observatory
X-rays trace hot (million degrees), highly ionized gas in the Universe.
Cosmic Rays
• Radiation from space does not only come in the form of electromagnetic radiation (radio, …, gamma-rays)
• Earth is also constantly bombarded by highly energetic subatomic particles from space: Cosmic Rays
• The source if cosmic rays is still not well understood.